A. Field of the Invention
The present invention relates generally to the preparation of surfactant-free dispersions comprising a low viscosity hydrophilic continuous phase and the use of such dispersions in topical formulations. In certain aspects, the present invention is directed to stable surfactant-free dispersions of hydrophobic substances in low viscosity hydrophilic glyceryl polyacrylate or glyceryl polymethacrylate fluids, and topical vehicles formulated with such ingredients.
B. Background
Topical formulations generally have to combine multiple, and sometimes competing, attributes, such as those relating to aesthetics, stability, delivery and compatibility. In regard to aesthetics, topical formulations are generally required to display appropriate tactile and visual properties when applied to the skin. Such formulations should also maintain functionality in regard to the delivery of various active agents onto or into the skin. Stability requirements apply both to individual ingredients (e.g., active agents) as well as to more gross aspects of the product form, such as precipitation or potential separation of phases. Compatibility constraints generally require that a formulation can be applied to the skin without eliciting a significant adverse reaction, such as an allergic or irritant response.
Formulations range from aqueous based solutions and gels to anhydrous vehicles. Simple aqueous based vehicles have limitations in regard to both solubility and stability of active agents, and can have poor tactile properties. Anhydrous vehicles have limitations in regard to desirable aesthetic properties. Emulsions are often employed in skin formulations to obviate such problems. An emulsion is a dispersion comprising two immiscible liquid phases, wherein one phase, a non-continuous phase, is dispersed into the continuous phase. The two basic types of such dispersions usually comprise either a hydrophilic, e.g. aqueous, dispersed phase in a hydrophobic continuous phase (water-in-oil) or a hydrophobic dispersed phase in a hydrophilic continuous phase (oil-in-water).
Emulsions are generally made by preparing separate hydrophobic and hydrophilic phases and mixing the two together. This is generally accomplished with the use of one or more emulsifying agents which reduce the surface tension between the immiscible phases creating micelles, i.e., a surfactant effect, thereby making the dispersion physically stable. However, surfactants have long been known to be associated with skin irritation and comprise the majority of adverse skin reaction to personal care products (e.g., Barany et al., 1999; Bodin et al., 2000; Effendy & Maibach, 1995).
Low viscosity dispersions are advantageous in that they can be readily processed through various mixers, mills and homogenizing equipment in order to produce a stable dispersion without the build up of excessive heat, which can occur when processing high viscosity “gel-like” dispersions due to friction. Low viscosity dispersions can spread and absorb easily into the skin and impart a characteristic “light” feel. However, the omission of surfactants from low viscosity dispersions would be expected to result in physical instability of the dispersed phase, i.e. phase separation, in contrast to high viscosity dispersions wherein the gel-like consistency physically stabilizes the dispersed particles.